A number of arrangements are known in the art for coupling signals from a bidirectional transmission facility to receive and transmit unidirectional transmission facilities. One example of this type transmission network is employed to couple a two-wire bidirectional telephone transmission facility to a four-wire telephone transmission facility. These arrangements are commonly referred to as hybrid circuits, whether they employ a hybrid transformer or not.
As is now known, it is desirable to adjust a complex impedance circuit in the transmission coupling network in order to obtain a better match to the impedance of the bidirectional transmission facility thereby maximizing loss between the receive and transmit unidirectional facilities. This is commonly referred to as maximizing transhybrid loss.
Many transmission networks employ so-called hybrid networks to realize the desired two-to-four wire coupling. Transmission networks employing either hybrid transformers or electronic circuits are now commonly employed in telephone transmission systems to realize the desired two-to-four wire coupling. In using either a hybrid transformer or an electronic "hybrid" it is desirable to employ a network having an impedance which substantially matches the impedance of the bidirectional transmission facility. Otherwise, low transhybrid loss results which, in turn, yields unwanted signal reflections. That is to say, a portion of the signal on the receive unidirectional facility appears in the transmit unidirectional facility. To this end, in transformer type hybrids, a complex impedance network is employed in an attempt at matching the impedance of the two-wire facility. Similarly, in electronic canceller type hybrids, a network having a complex transfer (impedance) characteristic is employed to generate an error signal in attempting to cancel unwanted signals in the transmit unidirectional facility.
In either arrangement adjustable impedance networks have been used in order to obtain a better impedance match to the bidirectional facility and, hence, to maximize transhybrid loss.
In transmission networks which employ hybrid transformers it has become the practice to employ an electronic network to generate a driving point impedance which emulates the complex impedance of the bidirectional transmission facility and, thereby, balance the hybrid transformer.
In a copending application of J. F. Rizzo and R. A. Rudisill, Ser. No. 06/064,041, filed Aug. 6, 1979, an arrangement is disclosed for automatically adjusting impedance elements of an adjustable impedance network in an attempt at obtaining an optimum match to a particular bidirectional transmission facility. The arrangement employs individual tones which are supplied to a receive port of a transmission network while corresponding individual impedance elements are adjusted until an amplitude null is detected at a transmit port of the transmission network. The tone signals are supplied and the amplitude adjustments are made in a prescribed sequence in order to obtain the best match to the impedance of the bidirectional facility. The disclosed procedure rapidly yields an optimum match for a nonloaded bidirectional facility. However, since the adjustable impedance network for a loaded bidirectional facility generates an impedance which approximates that of the loaded facility, it is possible that the individual tone signals may be at frequencies at which impedance peaks or valleys may occur when attempting to match the loaded bidirectional facility. Consequently, this results in less than an optimum match to the impedance of the loaded facility.
Thus, although the prior known balance arrangements are satisfactory for some applications they are undesirable for others.